This invention relates to castable Ni-base heat-resistant alloys suitable for use as materials that form the rotating blades and stationary vanes of a gas turbine, and other machine parts that are to be subjected to elevated temperatures.
Nickel-base heat-resistant alloys that are predominantly used as constituent materials for producing the rotating blades and stationary vanes of a gas turbine, the moving vanes of a hot blower and other machine parts that are to be subjected to elevated temperatures are those which are both precipitation hardened with the .gamma.' phase {Ni.sub.s (Al, Ti)} and solid-solution hardened with Mo, W, etc. See, for example, Japanese Patent Publication No. 59344/1989 which describes a Ni-base heat-resistant alloy that has high strength and high resistance to oxidation and corrosion at elevated temperatures and which consists, by weight percent (all percentages that follow are on a weight basis), of 7-13% Cr, no more than 35% Co, no more than 8% Mo, no more than 3% Nb, no more than 14% W, no more than 6% Ta, 4-7% Al, 0.5-6% Ti (provided Al+Ti=6.5-10.5%), no more than 1.5% V, no more than 0.2% Zr, 0.7-5% Hf, 0.02-0.5% C and 0.002-0.2% B, with the remainder being Ni and incidental impurities. If the addition of Mo, W, etc. to those alloys is excessive, deleterious phases such as the .alpha. and .mu. phases will develop and, hence, Al and Ti are added in large amounts so that more of the .gamma. phase will develop to give higher strength at elevated temperatures.
In such predominant Ni-base heat-resistant alloys, Mo and W are added in large amounts to an extent that will not cause the formation of any deleterious phases in the alloy structure and this inevitably limits the Cr content to 7-13%. Under the circumstances, the high-temperature strength of the alloys is improved but, on the other hand, their resistance to oxidation and corrosion at elevated temperatures is so much reduced that the alloys can only be used as constituent materials for fabricating gas turbines of a type that operates on high-grade fuels which emit smaller amounts of oxidizing and corrosive materials upon combustion. It has therefore been required to develop Ni-base heat-resistant alloys that can be used as constituent materials for fabricating gas turbines of a type that can produce a higher output power even if they are operated on low-grade fuels.